Seatbelt restraint systems for restraining an occupant in a vehicle seat play an important role in reducing occupant injury in vehicle crash situations. Seatbelt restraint systems of the conventional so-called “3-point” variety commonly have a lap belt section extending across the seat occupant's pelvis and a shoulder belt section crossing the upper torso, which are fastened together or are formed by a continuous length of seatbelt webbing. The lap and shoulder belt sections are connected to the vehicle structure by anchorages. A belt retractor is typically provided to store belt webbing and may further act to manage belt tension loads in a crash situation. Seatbelt restraint systems which are manually deployed by the occupant (so-called “active” types) also typically include a buckle attached to the vehicle body structure by an anchorage. A latch plate attached to the belt webbing is received by the buckle to allow the belt system to be fastened for enabling restraint, and unfastened to allow entrance and egress from the vehicle. Seatbelt systems, when deployed, effectively restrain the occupant during a collision.
OEM vehicle manufacturers often provide seatbelt restraint systems with pretensioning devices, which tension the seatbelt either prior to an impact of the vehicle or even prior to impact (also known as a “pre-pretensioner”) to enhance occupant restraint performance. The pretensioner takes out slack in the webbing and permits the belt restraint system to couple with the occupant early in the crash sequence. One type of pretensioner acts on the webbing retractor to tension the belt. Various designs of retractor pretensioners presently exist, including a type known as a roto-pretensioner that incorporates a gas generator for generating a pyrotechnic charge. Examples of such roto-pretensioners are described in U.S. Pat. No. 5,881,962, filed Apr. 11, 1995, U.S. patent application Ser. No. 11/115,583, filed Apr. 27, 2005, and U.S. patent application Ser. No. 12/830,792, filed Jul. 6, 2010, which are commonly owned by the assignee of the present application and are hereby incorporated by reference in their entirety for all purposes. Generally, ignition of the pyrotechnic charge or other combustible material creates gas pressure in a chamber having a piston to impart motion upon a driving element such as a piston, rack and pinion, or series of balls disposed in a pretensioner tube, which engage with and wind a retractor spool sprocket to retract the webbing.
One issue with pretensioners using a series of metallic balls is the weight of the series of balls required for a full pretensioning stroke, as well as the corresponding cost of supplying multiple metallic balls with strict tolerances. Further, for pretensioners using a series of metallic balls, or rack and pinion based systems, is the need for a synchronizing or clutch feature to ensure that the series of balls or pinion sufficiently engage the retractor spool sprocket.
Another issue with pretensioners is known as a low resistance condition, where the driving elements will reach an end of stroke without experience substantial resistance. This can occur if there is excessive slack in the seatbelt webbing. In these cases, the low resistance results in a lower amount of backpressure from the driving elements. The backpressure is produced by the engagement between the driving elements and the sprocket, so lower backpressure reduces the pressure on a sealing element that trails the driving elements. Reduced pressure on the sealing elements reduces the amount that the sealing element is compressed circumferentially. Reduced sealing ability can cause gas to leak from the tube around the series of balls.
A further issue with pretensioners is the need to maintain the retractor and the seatbelt webbing in a locked condition at the end of the pretensioning stroke. When the retractor spool does not remain locked, payback can occur which allows the seatbelt to unspool and reintroduce slack in the seatbelt. One method for maintaining the locked position includes maintain pressure from the gas generator beyond the amount needed for the pretensioning stroke. However, this adds weight and cost.